1. Field of the Invention
The present invention concerns a magnetic resonance imaging method based on the application of the TrueFISP sequence and the simultaneous acquisition of multiple parallel slices of a measurement subject. The invention also concerns a magnetic resonance tomography apparatus for implementation of such a method.
2. Description of the Prior Art
Magnetic resonance tomography (MRT) is an imaging modality with great importance for medical diagnostics. The continuous development of new or optimized pulse sequences significantly contributes to the success of MRT. Such new development is primarily intended to shorten the measurement time and/or to attain a higher contrast and/or a better signal/noise ratio (SNR).
Spin echo sequences were initially developed that partially exhibit the disadvantage of relatively long measurement times. Short measurement times can be obtained with gradient echo sequences. The FLASH sequence (“Fast Low Angle Shot”) represents the classical gradient echo sequence but exhibits the disadvantage that the coherent transverse magnetization is destroyed with a spoiler pulse after the signal detection. In the TrueFISP sequence (“Fast Imaging with Steady State Precession”), which is also designated as the b-SSFP sequence (“balanced Steady State Free Precession”), the coherent magnetization is not destroyed. Instead, a rephasing of the transverse magnetization is implemented after the radio-frequency excitation of a slice and the signal detection, allowing high signal intensities to be achieved. The TrueFISP sequence on which the present invention is based was first described by Oppelt et al. (Oppelt, A.; Graumann, R., Electromedica 54 (1), p. 15-18 (1986); U.S. Pat. No. 4,769,603). For a comprehensive depiction of the TrueFISP sequence as it is applied in the present invention, the article by Scheffler K. et al. is referenced (Scheffler K.; Lehnhardt S., Eur. Radiol. (2003) 13:2409-2418).
The inventive MR imaging method enables the measurement of a number of slices and representation under application of the 2D Fourier reconstruction methods and the measurement of a number of volumes and representation using the 3D Fourier reconstruction methods. In both cases the multi-slice technique essential to the invention is applied.
In the case of the 2D Fourier reconstruction, an individual data acquisition step of the TrueFISP sequence (i.e. the acquisition of the data for a phase coding line) can be divided into four time segments I through IV as is known in the art. In time segment I, the protons are excited with a radio-frequency pulse (RF pulse) at a flip angle α, and the excitation ensues slice-selectively since a slice selection gradient +GS is activated temporally in parallel for excitation during a time span 2T. In time segment II, a slice selection gradient −GS, a phase coding gradient +GP and a readout gradient −GR are activated. The (switching) activation of these three gradients ensues over a time span T. In the time segment III, a readout gradient +GR of the duration 2T is activated. During this activation, the magnetization is completely re-phased up to the point in time T at which the gradient echo is read out (i.e. the signal is detected). After the entire time interval 2T, the magnetization is de-phased again. In the time segment IV, a slice selection gradient +GS, a phase coding gradient −GP and the readout gradient −GR that is essential for the TrueFISP sequence are respectively activated over a time span T. The data acquisition for a phase coding line ends at the end of the time segment IV. Due to the pulse sequence described above for the TrueFISP sequence, which proves to be highly symmetrical in the graphical representation (see Scheffler K. et al, as cited above [I.c.]), the magnetization after the time segment IV is completely re-phased, and the magnetization is altered somewhat as a consequence of a certain T1 and T2 relaxation. The next α RF excitation pulse is subsequently radiated for the data acquisition for the next phase coding line, with the algebraic sign of α being changed and the algebraic sign of the gradient remaining unchanged. The time span from one α RF pulse to the next α RF pulse corresponds to what is known as the repetition time TR and represents a TR interval. The above cycle is repeated MA times corresponding to the number MA of the phase coding lines of the raw data matrix. The total measurement of a slice by application of the TrueFISP sequence accordingly takes MA·TR.
In the case of data acquisition of volumes with 3D Fourier reconstruction, a phase coding gradient GS,P is additionally activated in the slice direction.
In the inventive method, the sequence of pulses, gradients and signal detection within a TR interval described above is used.
It can be shown that the highly-symmetrical pulse sequence and the re-phasing of the magnetization resulting therefrom lead to the adjustment of a dynamic equilibrium state of the magnetization after a larger number of TR intervals, this dynamic equilibrium state being designated as a “steady-state free precession” SSFP (see Scheffler K. et al., cited above). Before reaching this equilibrium state, the magnetization passes through a transient range in which the magnetization is significantly higher than in the equilibrium state. Since the magnetization in this time interval exhibits strong fluctuations, however, the transient range cannot be used for the data acquisition without further measures. The fluctuations disappear almost completely when an α/2 RF excitation pulse (also designated as an α/2 RF preparation pulse in the following) is radiated into the measurement subject with a time interval of TR/2 before the first α RF excitation pulse (Deimling, M.; Heid, O.; Society of Magnetic Resonance p. 495, 1994, Proceedings). Both the transient range and the subsequent dynamic equilibrium state then can be used for the data acquisition.
In the dynamic equilibrium state the TrueFISP sequence enables the highest possible SNR per time unit of all known sequences. However, it is problematic that the signal intensity in the dynamic equilibrium state is proportional to the quotient T2/T1 from the spin-spin relaxation constant (T2) and the spin-grid relaxation constant (T1). For data acquisition in the dynamic equilibrium state, the high signal intensity and the high SNR are achieved only for materials with similarly large T2 and T1. This applies to blood and cerebrospinal fluid (CSF). In contrast, tissues in which T1 is typically significantly larger than T2 are shown signal-poor. Examples of this are the white and grey brain matter, the liver tissue and the musculature. In contrast, in the transient range the signal intensity is more strongly proton density-weighted and barely dependent on the T2/T1 quotient, such that for this reason the transient range is also of interest for MR imaging with regard to the tissue shown signal-poor.
For application of the TrueFISP sequence with the typical linear phase coding, due to the T2/T1 weighting, MR images are acquired in which blood and CSF are mapped with high signal intensity and high SNR and the aforementioned tissues are mapped With low signal intensity and poor SNR. This result is not improved by the transient range for the data acquisition being made accessible by radiation of an α/2 RF pulse because, given the linear phase coding, the data entries in the central k-space lines, that are important for the SNR and the contrast, are acquired only after filling half of the raw data matrix. At this point in time the excited slice has already reached the dynamic equilibrium state with its lower magnetization and the T2/T1 weighting of the signal intensity.
To solve this problem it has been proposed to implement the TrueFISP sequence with centrically-arranged phase coding and α/2 RF preparation pulse. Centric phase coding means that initially the central lines of k-space and subsequently, successively the phase coding lines above and below these central lines are measured. The significantly earlier measurement of the central k-space lines actually leads to a distinct improvement of the SNR and of the contrast in the region of the aforementioned tissue. A disadvantage is that structures in the resulting MR image are shown blurred.
It is furthermore known to use the TrueFISP sequence with linear phase coding for the sequential measurement of a number of slices of a measurement subject. Sequential measurement means that the complete data acquisition is implemented for a slice before the next slice is measured. Since the workflow of the measurement for each individual slice hereby remains unchanged, the same problems exist as for the measurement of only one slice. This applies in the same manner when the sequential measurement of a number of slices is implemented using the centric phase coding. The problems occurring in connection with the measurement of only one slice also still exist.